E L S E V I E R Inorganica Chimica Acta 268 (1998) 263-269

Structure and magnetic properties of a linear oximato-bridged tetranuclear copper(II) complex

Rafael Ruiz a, Francesc Lloret a.., Miguel Julve a, Juan Faus a, M. Carmen Mufioz b, Xavier Solans c

a Departament de Qu(mica Inorgdnica, Universitat de Valencia, Dr Moliner 50, 46100 Burjassot, Valencia, Spain b Departament de Ffsica Aplicada, Universidad Polit~cnica de Valbncia, Camino de Vera s/n, 46071 Valbncia, Spain

Departament de Cristal.lografia i Mineralogia, Universitat de Barcelona, Mart{ i Franqu~s s/n, 08028 Barcelona, Spain

Received 11 March 1997; revised 21 April 1997; accepted 22 May 1997

Abstract

The tetranuclear copper(II) complex of formula [Cu2(dmg) (Hdmg) (terpy) ] 2(C104)2 (1) (Hzdmg = dimethylglyoxime and terpy = 2,2':6',2"-terpyridine) has been synthesized and its crystal structure determined by X-ray diffraction methods. It crystallizes in the triclinic system, space group P ( - I ) , with a=14.382(3), b=13.728(3), c=8.979(2),~, a=96.99(2), fl=111.85(2), y=111.22(3) °, V = 1465.0 ( 9 ) ,~3, Z = 1, Dc = 1.607 g cm - 3, Mr = 1418.0, F(000) = 719, A (Mo Kot ) = 0.71073/k,/x ( Mo Ka ) = 16.61 cm - t and T = 298 K. A total of 4891 reflections were measured over the range 2 _< 0_< 25 ° and 4393 of them were unique (I > 2.5 o-(l) ) and used in the structural analysis. The structure of 1 may be described as two dimeric [Cu2(dmg)(Hdmg)(terpy)] + units linked by relatively long out-of-plane oxime bonds building a cationic centrosymmetric tetrameric entity with a linear Cu4 (NO) 8 core, and uncoordinated perchlorate anions. Within each dinuclear unit, the partially deprotonated [ Cu (dmg)(Hdmg) ] - complex unit coordinates to the [ Cu(terpy) ] 2 + fragment through in- plane oxime bonds in an asymmetric fashion. The coordination geometry around both copper atoms is square pyramidal. The intra- and interdimer copper separations within the tetranuclear complex are 3.674(5) and 3.994(6) A, respectively. Variable-temperature magnetic susceptibility measurements (4.2-300 K) have been interpreted on the basis of the tetranuclear nature of the compound. The nature and magnitude of the magnetic couplings are discussed on the basis of the structural features and compared with that of related oximato-bridged copper(II) dinuclear complexes. © 1998 Elsevier Science S.A.

Keywords: Crystal structures; Magnetic properties; Copper complexes; Tetranuclear complexes; Oximato-bridged complexes

1. Introduction

Polynuclear transition metal complexes have played a cru- cial role in the field of molecular magnetism [ 1,2]. So, rel- evant concepts in molecular ferromagnetism [3] such as the strict orthogonality of magnetic orbitals or the irregular spin- state structure appear to be closely associated with the study of dinuclear copper ( I I ) -vanadium(IV) [4] and trinuclear manganese( I I ) -copper ( I I ) -manganese( I I ) complexes [5], respectively. In this line of research, it seems reasonable to assume that new basic concepts will emerge from the study of higher nuclearity complexes such as the tetranuclear com- plexes. Among others, compounds of this kind exhibit some general features such as spin frustration [ 6] or diverse molec- ular topology [ 7 ]. For instance, the small number of tetra-

nuclear copper(II) complexes reported so far [ 8 ] includes

* Corresponding author.

0020-1693/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved P I I S 0 0 2 0 - 1 6 9 3 ( 9 7 ) 0 5 7 5 5 - 1

a variety of molecular structures such as those shown in Scheme 1 where the low-lying spin state is mainly dictated by the topology, e.g. linear [8a-c] , square [8d] or rhombic

[8e-h] . Our research group is particularly interested in the study

of the alternating magnetic exchange interactions in poly- nuclear systems. In this context, we have characterized cop- per(II) chains with regular alternation of antiferromagnetic

Scheme 1.

264 R. Ruiz et al. / lnorganica Chimica Acta 268 (1998) 263-269

C N N 0 d lcu

interactions (through oxamidato or bipyrimidine bridging units) and ferromagnetic interactions (through double hydroxo or end-on azido bridging ligands) [9,10], as well as sheet-like polymers with first-row transition metal ions which behave as alternating magnetic planes (antiferromag- netic coupling through bipyrimidine and oxalato ligands and ferromagnetic coupling through double end-on azido groups) [ 11,12]. In these examples, the alternation of the bridging ligands provides an easy way to obtain a variety of magnetic systems with alternating ferro- and antiferromagnetic inter- actions. However, other strategies are possible in order to achieve this goal. In that sense, oximato-bridged copper(II) dinuclear complexes are of great interest from both structural and magnetic viewpoints. The deprotonated oxime group is very versatile [ 13,14], and it can adopt either in-plane (I) or an out-of-plane ( I I ) coordination mode when acting as a diatomic (/z-l,2) bridging ligand between copper(II) ions.

Cu

Cu

I II The magnetic coupling is strongly antiferromagnetic in the former case [15-19] and weakly antiferro-[20] or ferro- magnetic [ 21 ] in the latter. This striking ability of the oxime group to stabilize either the singlet or the triplet ground state in oximato-bridged copper(II) dinuclear complexes has recently been investigated by some of us [22] and explained in the framework of the molecular orbital model proposed by Hay et al. [23]. On the basis of these studies, a magneto- structural correlation between the magnitude of the magnetic coupling in out-of-plane copper(II) dimers and the value of the angle at the out-of-plane oxime to copper bond (o0 or the metal-to-ligand axial distance (R) has been established [221.

In the present work, we report the preparation and the structural and magnetic characterization of the tetranuclear copper(II) complex of formula [Cuz(dmg)(Hdmg)- (terpy) ] 2 (C104) 2 ( 1 ) (H2dmg = dimethylglyoxime; terpy = 2,2':6',2"-terpyridine), where for the first time both in- plane and out-of-plane doubly oximato-bridged copper(II) pairs are present in the same complex. Furthermore, com- pound 1 provides us with the possibility of analysing the influence of a series of factors such as the alternation of antiferro- and ferromagnetic interactions or the molecular topology on the nature of the low-lying spin state.

2. Experimental

2.1. Reagents

Copper(II) perchlorate hexahydrate, terpy and H2dmg were obtained from commercial sources and used without

further purification. The bis(dimethylglyoximato)copper- (II) complex [Cu(Hdmg)2] was prepared as described pre- viously [ 18a]. Elemental analyses (C, H, N) were performed by the Microanalytical Service of the Universidad Aut6noma de Madrid (Spain).

2.2. Preparation of l Cue(dmg)( Hdmg)(terpy)]e( ClO4)e (1)

A mixture of [Cu(Hdmg)2] (294 mg, 1 mmol), terpy (233 mg, 1 mmol) and Cu(CIO4)2.6H20 (371 mg, 1 mmol) in methanol (50 cm 3) was stirred under reflux for 2 h. Black rhombic prisms of 1 suitable for single-crystal X-ray analysis were obtained after 3 days by slow evaporation of the filtered solution at room temperature. The crystals were collected by filter suction and air-dried. Anal. Found: C, 39.75; H, 3.34; N, 13.77. Calc. for C23H24C1N708Cu2: C, 40.09; H, 3.49; N, 14.23%. IR data u,,ax (cm-~) : 1549s(sh) (CN), 1205 and 1195vs(sh) (NO) and 2600w(br) (OH-.-O).

2.3. Physical techniques

The IR spectrum was recorded on a Perkin-Elmer 1750 FTIR spectrophotometer as KBr pellets. Magnetic suscepti- bility measurements were carried out in the temperature range 4.2-300 K with a fully-automated AZTEC DSM8 pendulum- type susceptometer [24] equipped with a TBT continuous- flow cryostat and a Bruker BE 15 electromagnet operating at 1.8 T. The apparatus was calibrated with Hg[Co(NCS)4]. Diamagnetic corrections to the susceptibility of complex 1 were calculated from Pascal's constants [25] as - 2 8 8 × 10 -6 cm 3 mol- ~. Magnetic susceptibility data were also cor- rected for the temperature-independent paramagnetism (60 × 10 -6 cm 3 per CuII ion) and the magnetization of the sample holder.

2.4. X-ray crystallography of Cu2(dmg)(Hdmg)- (terpy)12( Cl04)2 (1)

2.4.1. Crystal data and data collection parameters C46H48CI2NI4016Cu4, M = 1418.07 g mol - J, triclinic, a =

14.382(3), b=13.728(3) , c=8.979(2)A, , a=96 .99 (2 ) , 13= 111.85(2), 3, = 111.22(3) °, V= 1465.0(9) .~3 (by least- squares refinement on diffractometer angles from 25 centred reflections, 8 < 0< 12°), T=298 K, space group P( - 1), graphite-monochromated Mo Kc~ radiation, A = 0.71069 ,~, Z = 1, Dc = 1.607 g c m -3 , F ( 0 0 0 ) =719.0, black rhombic prism with dimensions 0 .2×0 .1×0 .1 mm, /z(Mo K a ) = 16.61 c m - ~, Lorentz and polarization effects but no absorp- tion corrections; Philips PW-1100 diffractometer, w scans, data collection range 2 < 0<25 °, three standard reflections monitored throughout data collection every 2 h showed no significant variation in intensity; 4981 reflections measured, 4393 of them were unique with 1> 2.5o-(1) and they were used in all calculations.

R. Ruiz et al. / lnorganica Chimica Acta 268 (1998) 263-269 265

2.4.2. Structure solution and refinement The structure was solved by direct methods (program

SHELX-86) [26] and refined by full-matrix least-squares methods (program SHELX-76) [27]. All non-hydrogen atoms were refined anisotropically. Two (O (6) and O ( 7 ) ) of the four oxygen atoms of the perchlorate anion were located in disordered positions, each atom being distributed in two peaks to which an occupancy factor of 0.50 was assigned according to the Fourier map height of the respective peak. All hydrogen atoms (24) were located from a differ- ence synthesis. The hydrogen atom linked to 0 ( 2 ) was refined without constraints and the remaining 23 were refined

Table 1 Final fractional atomic coordinates a and equivalent isotropic displacement parameters u for non-hydrogen atoms of 1

Atom x /a y /b z /c Ueq (A, 2)

Cu(l) 0.03001(6) 0.36470(6) 0.32868(8) 3.06(3) Cu(2) 0.08549(6) 0.13714(6) 0.19212(1) 2.95(3) C1 -0.4011(2) -0.2644(2) 0 .3028(3) 6.53(11) N(I) -0.1148(4) 0 . 3179 (4 ) 0 .1131(7) 3.69(23) N(2) 0 . 0 3 9 3 ( 4 ) 0 . 5082 (4 ) 0 .3074(6) 3.23(21) N(3) 0 . 1 5 9 5 ( 5 ) 0 . 4670 (4 ) 0 .5584(6) 3.57(23) N(4) 0 . 1 7 8 6 ( 4 ) 0 . 2921 (4 ) 0 .2267(6) 3.10(21) N(5) 0 . 2 1 3 5 ( 4 ) 0 . 1261 (4 ) 0 .1729(6) 3.28(22) N(6) -0.0107(3) 0 . 1426 (4 ) 0 .3022(6) 2.93(21) N(7) 0 .0100(4) -0.0190(4) 0 .1716(6) 3.14(21) O(1) 0 . 1 4 8 4 ( 4 ) 0 . 3735 (3 ) 0 .2346(5) 3.43(18) 0(2) 0 . 2 2 3 8 ( 4 ) 0 . 0294 (4 ) 0 .1415(7) 4.81(24) 0(3) -0.0106(4) 0 . 2319 (3 ) 0 .3875(5) 3.48(18) 0(4) 0 .0273(4) -0.0977(3) 0 .0923(5) 3.36(18) 0(5) -0.2901(5) -0.2264(6) 0 .4224(8) 7.39(34) 0(6) -0.4172(17) -0.1943(16) 0.2138(24) 11.07(93) 0(6") -0.4094(29) -0.1655(31) 0.3078(83) 30.54(356) 0(7) -0.4654(28) -0.3526(28) 0.3003(53) 15.61(215) 0(7") -0.4737(16) -0.3180(23) 0.3748(27) 9.61(132) 0(8) 0 .4176(17) 0.3485(18) 0.8370(18) 12.84(123) C(I) -0.1915(6) 0 . 2165 (7 ) 0 .0249(9) 5.22(34) C(2) -0.2862(7) 0 .1963(8) -0.1277(11) 6.39(42) C(3) -0.2923(7) 0 .2834(9) -0.1818(10) 5.99(43) C(4) -0.2138(7) 0 .3852(7) -0.0948(8) 4.95(36) C(5) -0.1237(6) 0 . 4039 (6 ) 0 .0573(8) 3.89(30) C(6) -0.0338(6) 0 . 5140 (6 ) 0 .1687(7) 3.46(28) C(7) -0.0240(7) 0 . 6112 (6 ) 0 .1427(9) 4.40(35) C(8) 0 . 0 6 1 2 ( 7 ) 0 . 7060 (6 ) 0.2609(10) 4.75(41) C(9) 0 . 1 3 9 7 ( 7 ) 0 . 7018 (6 ) 0 .4133(9) 4.38(33) C(10) 0 .1241(6 ) 0 . 5982 (5 ) 0 .4294(7) 3.50(27) C(ll) 0 .1932(5 ) 0 . 5741 (5 ) 0 .5714(7) 3.36(25) C(12) 0 .2879(6 ) 0 . 6553 (6 ) 0 .7087(8) 4.46(31) C(13) 0 .3494(6 ) 0 . 6 2 2 2 ( 8 ) 0 .8319(9) 5.49(37) C(14) 0 .3180(6 ) 0 . 5 1 3 4 ( 7 ) 0 .8163(9) 4.94(34) C(15) 0 .2200(7 ) 0 . 4 3 6 6 ( 7 ) 0 .6783(8) 4.45(34) C(16) 0 .2767(5 ) 0 . 3 1 3 7 ( 5 ) 0 .2346(8) 3.46(25) C(17) 0 .3576(6 ) 0 . 4 2 5 3 ( 7 ) 0.2685(12) 6.32(42) C(18) 0 .2960(5 ) 0 . 2 1 6 2 ( 6 ) 0 .2072(8) 3.68(29) C(19) 0.407l(8) 0 . 2239 (9 ) 0.2154(14) 6.64(52) C(20) -0.0719(5) 0 . 0 4 7 7 ( 5 ) 0 .3077(7) 2.98(23) C(21) -0.1478(6) 0 . 0 3 5 7 ( 6 ) 0 .3901(9) 4.32(31) C(22) -0.0624(5) -0.0451(5) 0 .2291(7) 2.98(23) C(23) -0.1317(6) -0.1621(5) 0 .2149(8) 3.87(28)

E.s.d.s in the last significant digits are given in parentheses. b Ucq is defined as eight third of the trace of the orthogonalized U~j tensor.

Table 2 Selected bond lengths (A.) and angles (o) a.b for complex 1

Copper( 1 ) environment Cu(1)-N(1) 2,056(5) Cu(I)-N(2) 1.963(6) Cu(1)-N(3) 2.059(4) Cu(1)-O(1) 2.139(6) Cu( 1 )-0(3) 1.907(5)

N(1)-Cu(I)-N(2) 7 9 . 5 ( 2 ) N(1)~Su(1)-N(3) 155.5(3) N(1)-Cu(1)-O(3) 9 8 . 0 ( 2 ) N(2)-Cu(1)-N(3) 78.7(2) N(2)-Cu(1)-O(3) 159.3(3) N(3)-Cu(1)-O(3) 98.6(2) O(I)-Cu(1)-N(1) 102.7(2) O(1)~u(1)-N(2) 93.0(2) O(1)-Cu(1)-N(3) 89.4(2) O( 1)-Cu(1)~O(3) 107.5(2)

Copper(2) environment Cu(2)-N(4) 1.970(5) Cu(2)-N(5) 1.969(7) Cu(2)-N(6) 1.993(7) Cu(2)-N(7) 1.969(7) Cu(2)~O(4 ~) 2.321 (4)

N(4)-Cu(2)-N(5) 8 0 . 1 ( 3 ) N(4)-Cu(2)-N(6) 104.0(2) N(4)~Cu(2)-N(7) 172.9(2) N(5)--Cu(2)-N(6) 156.6(2) N(5)~2u(2)-N(7) 94.0(2) N(6)-Cu(2)-N(7) 80.0(2) O(4~)-Cu(2)-N(4) 95.8(2) O(41)-Cu(2)-N(5) 97.6(2) O(4~)-Cu(2)-N(6) 96.7(2) O(4~)-Cu(2)-N(7) 88.7(2) Cu(2~)-O(4)-N(7) 88.8(3)

a E.s.d.s in the last significant digits are given in parentheses. "Symmetry code (I) -x, -y, -z.

with an overall isotropic factor using a riding model. The function minimized was Ew( I F o l - Ifcl )2 (Fo and Fc are the observed and calculated structure factors) with w = 0"2( I Fol + 0.0085 lEo 12) - i ; f , f , andf" were taken from the literature [28]. The final R (Rw) factor was 0.064 (0.071), for 391 refined parameters; maximum A/o-=0.1 , maximum A p = 0 . 4 e , ~ -3, minimum A p = - 0 . 4 e , ~ -3. The final coordinates for non-hydrogen atoms and selected bond distances and angles are listed in Tables 1 and 2.

3. Results and discussion

3.1. Description of the structure of 1

The crystal structure of 1 consists of centrosymmetric tetranuclear cations [Cu2(dmg)(Hdmg)( terpy)]2+ and uncoordinated perchlorate anions. A perspective view of the asymmetric dinuclear unit with the atomic numbering scheme is shown in Fig. l (a ) . The partially deprotonated [ C u ( d m g ) ( H d m g ) ] - complex coordinates to the [Cu- (terpy) ]2+ fragment through its cis oximate oxygens in an asymmetric fashion affording the dinuclear skeleton. Two symmetry-related dinuclear cations [Cu2(dmg)(Hdmg)- (terpy) ] ÷ which are linked by relatively long out-of-plane bonds through the Cu(2) and Cu(2 I) atoms ( I = - x , - y , - z ) compose the corresponding tetranuclear entity as depicted in Fig. l (b ) . This same kind of parallel-planar basal planes arrangement was previously observed in [Cu(Hdmg)2]2 [21]. In fact, this structure can be alterna- tively viewed as a central [ Cu (dmg) (Hdmg) ] 22- unit which acts as bis-chelating ligand towards two [ Cu (terpy) ] 2 + frag-

266 R. Ruiz et al. / lnorganica Chimica Acta 268 (1998) 263-269

C(3) C(2)

C(5) @ ~ N ( 1 ) C(23) O(4)

c(12) ~ c ( 1 4 )

(a) c~13)

(b) Fig. 1. (a) Perspective view of the asymmetric dinuclear unit [Cu2- (Hdmg) (drag)(terpy) ] ÷ of complex 1 with the atom labelling. Thermal ellipsoids are plotted at 30% probability level. Hydrogen atoms have been omitted for clarity. (b) View of the resulting tetranuclear unit obtained by polymerization through the out-of-plane oxime bonds.

ments to make up the corresponding tetranuclear entity. The packing of the tetranuclear complexes is illustrated by means of a stereoscopic view (Fig. S1, supplementary material).

The coordination geometry around the two crystallograph- ically independent copper atoms is distorted square-pyrami- dal, Cu( 1 ) N 3 0 2 and C u ( 2 ) N 4 0 : three imine-nitrogen atoms from the terpy ligand and one oximate-oxygen atom at Cu( 1 ) and four oxime-nitrogen atoms of the dimethylglyoxime groups at Cu(2) define the corresponding basal planes; the fifth apical position of the square pyramids for both copper atoms being occupied by oximate-oxygen atoms. The three Cu(1)-N(terpy) bond distances are distributed into two groups, that formed by the nitrogen atom of the central pyr- idine ring of the terpy ligand, Cu(1) -N(2) = 1.963(6) A, being shorter than those of the terminal ones, Cu(1 ) - N( 1 ) = 2.056(5) ,~ and Cu( 1 ) -N(3) = 2.059 (4) ,~, owing

to the geometrical constraints of the terpy ligand [29]. The two Cu( 1 ) -O (oxime) bond distances are markedly different from each other, the equatorial distance Cu(1 ) - 0 ( 3 ) = 1.907(5) ,~ being shorter than the axial one Cu( 1)- O(1)=2 .139(6) A owing to the Jahn-Teller effect. The Cu(2)-N(oxime) equatorial bonds lie in the range 1.97- 1.99 ,~ as found in related complexes [ 15-21,23,30-33]. The long axial Cu(2) -O(4 ~) bond distance 2,321(4) A is close to the corresponding Cu-O(oxime) distance in [Cu(Hdmg)2]2 (2.301 ~.) [21 ]. The four equatorial donor atoms around Cu( 1 ) and Cu(2) are nearly coplanar (maxi- mum deviations are 0.049,~ at N(2) around Cu(1) and 0.160 A at N(5) around Cu(2)) , with both copper atoms slightly displaced from their mean least-squares basal planes, 0.306 and 0.238,~ for Cu( l ) and Cu(2), respectively, towards the axial position. The dihedral angle between these basal planes is 79.8(2) ° . The larger deviations from the ideal value of 90 ° observed in the angles subtended at the copper atoms are due to the occurrence of five-membered (79.5 (2), 78.7(2), 80.1(3) and 80.0(2) ° for N (1 ) -Cu (1 ) -N(2 ) , N (2 ) -Cu (1 ) -N (3 ) , N ( 4 ) - C u ( 2 ) - N ( 5 ) and N ( 6 ) - Cu(2) -N(7) , respectively) and six-membered chelate tings (107.5(2) and 104.0(2) ° for O (1 ) -Cu (1 ) -O(3 ) and N (4 ) -Cu (2 ) -N (6 ) , respectively).

Charge-balance considerations dictate that there is one oxime proton per dinuclear unit (i.e. per two dimethylgly- oximate ligands). In the light of the reported structural data for dimethylglyoxime-containing copper(II) complexes [ 18,31a,32a], it may be anticipated that the O( 1 ) and 0 (3 ) oxime-oxygens are deprotonated and consequently the oxime-hydrogen atom might be attached to the O(2) or O(4) atoms. In fact, the short O(2)- . -O(4) separation (2.56(1) A) is consistent with the occurrence of a hydrogen bond between these two oxygen atoms. In this respect, the corre- sponding oxygen-oxygen distance in [Cu(Hdmg) 2] 2 is very close (2.526 and 2.684 A) [21]. The dmg 2- and Hdmg- ligands coordinate simultaneously in a bidentate fashion through their oxime-nitrogen atoms to Cu(2), and in a mon- odentate fashion through one oxime-oxygen atom to Cu ( 1 ), thus leading to the formation of alternating five- and six- membered chelating tings around Cu(2). Each five-mem- bered ring is almost planar (largest deviations are 0.049 and 0.016 ~, at N(5) and C(22), respectively). The average value of the carbon-nitrogen bond distances ( 1.289 A) for both drag z- and Hdmg- ligands agrees with that expected for a double bond. Interestingly, the nitrogen--oxygen bond lengths are grouped in two sets, with the N (5 ) -O (2) distance ( 1.395(9) A) being larger than the others (average value of 1.358 A). This shortening reflects a greater percentage of double bond character for the later set of bonds indicating that its oxime oxygen is deprotonated, and consequently it can be concluded that the lost oxime-hydrogen atom belongs to 0 (2 ) . The pyridyl tings of the tridentate te~y ligand are highly planar (maximum deviation is 0.0146 A at C( 11 )) , but the ligand as a whole is far from being planar. The dihedral angles between the two outer pyridyl tings and the central

R. Ruiz et al. / lnorganica Chimica Acta 268 (1998) 263-269 267

ring are 4.6(8) and 2.9(9) ° for N ( 1 ) C ( 1 ) C ( 2 ) C ( 3 ) - C(4)C(5) and N(3 )C(11)C(12)C(13)C(14)C(15) , respectively.

Finally, the most noticeable feature of the structure of complex 1 is the occurrence ofa C u 4 ( N O ) 8 bridging network as illustrated in Fig. 1 (b). The Cu( 1 ) and Cu(2) atoms are joined in an asymmetric fashion through two in-plane oxi- mate groups in a cis arrangement. The six-membered Cu( 1 ) O ( 1 ) N ( 4 ) C u ( 2 ) N ( 6 ) O ( 3 ) ring exhibits a boat con- formation, with Cu(1) and Cu(2) being 0.171 and 0.164 ~, out of the mean plane. As far as the central bridging network is concerned, the Cu(2) and Cu(2 ~) are doubly bridged by two out-of-plane oximate groups in a trans arrangement. The Cu(2)O(4r)N(7I)Cu(2~)O(4)N(7) ring adopts a chair conformation, with Cu(2) and Cu(2 ~) being 0.207 ]~ above and below of this mean plane. The Cu(2 r ) -O(4 ) -N(7 ) bridging angle (ee) is equal to 88.8 °, a value closer to the ideal value of 90 ° and far from that found in [Cu(Hdmg)2] 2 (102.5 °) [21]. The intramolecular Cu(1)- . -Cu(2) and Cu(2)- . .Cu(2 I) separations are 3.674(5) and 3.994(6) A, respectively, the greater one corresponding to the central di- /x-oximate-bridged copper(II) pair. This value is somewhat greater to that found in [Cu(Hdmg)2]2 (3.849 ~i) [21], reflecting thus the slight differences concerning the axial Cu- O (R) distances and the Cu-O-N (c~) angles for both com- pounds. The shortest intermolecular Cu( 1)---Cu(1") sepa- ration is 5.052(5) A ( I I = - x , 1 - y , 1 - z ) .

3.2. Magnetic properties

The magnetic behaviour of complex 1 is shown in Fig. 2 in the form of both the XM and xMT versus T plots, X~ being the magnetic susceptibility per four copper(II) ions and T the temperature. XMT at room temperature is 1.10cm 3 K mol - t , a value which is significantly lower than that expected for four uncoupled copper(II) ions. Upon cooling, XMT continuously decreases, vanishing at around 25 K. Moreover, XM shows a maximum at 170 K. This magnetic behaviour is characteristic of an overall antiferromagnetic

0.00S . . . . . . . . . . . . . . . . . . . . . . . , , . 1 . 2

o.oo4 1.o 0

~'~ 0.003 0.8 ~:~

t..) 0.002 0 "~" 0.4 "~

o.ooi 0.2

0.000 -- 0.0 50 100 150 200 250 300

T / K Fig. 2. Thermal dependence of the molar susceptibility for complex 1 in the form of XM (O) and xMT (O) vs. Tplots.

interaction between the Cu n ions in 1 with a singlet ground state.

We have interpreted the magnetic data according to the actual tetranuclear nature of the complex with the spin Hamiltonian/4= - J (S t ' 5~2 + $3" $4) - J'S2" $3 in which J is the exchange coupling constant between terminal and central coppers and J' is that between the central coppers (see Fig. 1 (b) ) . The resulting spin states and their corresponding energies are St = 2 $2 = 1 $3 = 1 S4 = 1 $5=0 56=0

E t = - J / 2 - J ' / 4

E 2 = J / 2 - J ' / 4 E3 =J' /4 + (j2 +j ,2) ,/2/2 Ea=J' / 4 - ( J2 + J'2) l/2 /2 E5 =J/2 + J ' / 4 + (4J 2 - 2JJ' +j,2) 1/2/2 Et=J/2 + J ' / 4 - (4 _ j 2 _ 2JJ' +j,2)1/2/2

The fit of the magnetic data to the appropriate susceptibility expression derived from the Van Vleck formula, assuming that the g factors for both copper(II) ions are identical [ 8b], leads to J = - 197 cm-~, J ' =28 cm - t and g=2 .03 (solid line in Fig. 2). The quality of the fit does not depend signif- icantly on the value of J ' . In fact, it is possible to fit the magnetic data to a simple Bleaney-Bowers equation [34] (that is J ' = 0 ) and the same value for J is obtained. The values J = - 197 cm-~, J ' =28 c m - , have been used (see below) to build the energy-level diagram shown in Fig. 3. The ground spin state is the singlet $5, the first excited states at energy ca. 200 c m - t are the two triplets $2 and $3 separated each other by an energy of the order of only 15 c m - ,. The highest states are the quintet St and the nearly degenerate triplet $4 and singlet $6 which are largely separated from the lower multiplet state by ca. 400 c m - t.

The magnitude of the antiferromagnetic coupling J agrees quite well with that found for the related oximato-bridged dinuclear compound [Cu2(pdmg)(terpy)](CIO4)2 ( J =

+200

-200

0

-400

S

Fig. 3. Energy-level diagram for complex I obtained from the analysis of the temperature dependence of the magnetic susceptibility according to the tetranuclear model (with J = - 197 cm ~ and J ' = 28 cm- ~ ).

268 R. Ruiz et al. /lnorganica Chimica Acta 268 (1998) 263-269

Table 3 Magneto-structural data for out-of-plane di-~-oximato-bridged copper(II) dinuclear units

Complex ot ~ (°) R b (A.) J ( c m - l ) Ref.

[Cu2(Hdmg) (dmg) (terpy) ]2} (CIOa)2 (1) [Cu(Hdmg)2]2} (2) [ Cu2Zn(Hdmg) 2(dmg)2(HzO) ] 2} (3) [Cu(Hchd)2]2} (4)c [Cu(Hdeg)2]2} (5) d [Cu(Hbdmg)]2} (CIO4)2 (6) e

88.8 2.321 28 this work 102.5 2.301 9.1 [21,22] 102.7 2.318 5.1 [36] 106.0 2.242 3.1 [ 22 ] 106.1 2.263 1.0 [22] 107.9 2.266 - 1.9 [ 22,30]

Angle at the out-of-plane oxime to copper bond. b Distance from the copper to the oxime oxygen atom. c Hzchd = 1,2-cyclohexadionedioxime. a H2deg = diethylglyoxime. e Hzbdmg = 3,10-dimethyl-4,9-diaza-3,9-dodecadiene-2,1 l-dionedioxime.

(2)

(b) Scheme 2.

- 1 7 4 c m - ~ ) , but differs from that of [Cu2(pdmg)- ( b i p y ) ( H 2 0 ) 2 ] ( C 1 0 4 ) 2 ( J = - 674 cm - I ) [H2pdmg = 3,3'-aminopropylenedinitrilobis (2-butanoneoxime), bipy =2,2'-bipyridine] [19]. The difference in the values of the magnetic coupling has been explained in terms of the well-known orbital reversal phenomenon caused by the pres- ence of terpy as terminal tridentate N-donor ligand [ 19,35]. This situation is illustrated in Scheme 2(a): the magnetic orbital of Cu( 1 ) is oriented perpendicular to that of Cu(2) such that overlapping occurs on only one side of the oxime bridge. The magnitude of the coupling J ' is expected to be much weaker than that of J, as observed. The magnetic orbi- tals of neighbouring central atoms Cu(2) and Cu(2 I) are parallel to each other in two different planes thus leading to a poor overlap as illustrated in Scheme 2 (b). As for the value of this weak interaction J ' between the central copper(II) ions in complex 1, it would be appropriate to mention here that we have previously reported a ferromagnetic interaction for the [ Cu (Hdmg) 2 ] 2 dinuclear complex, where the singlet- triplet energy gap is equal to 9.1 cm-~ [22]. We have also shown that the exchange interaction in this kind of complexes depends on some structural parameters such as the angle at the out-of-plane oxime to copper bond ( a ) or the copper axial bond distance (R). The overlapping between the mag- netic orbitals of the copper(II) ions diminishes as the values of c~ and R decrease in such a way that a ferromagnetic interaction is obtained for ~ values below 106.9 ° [22]. Both 1 and [Cu(Hdmg)2]2 have similar values for R (2.321 and 2.301 A, respectively), but the value of ce for 1 is smaller than that for [Cu(Hdmg)2]2 (88.8 versus 102.5 °, respec- tively). Consequently, a greater ferromagnetic interaction is expected for 1. Selected magneto-structural data concerning

Scheme 3.

the series of dimeric copper(II) complexes with out-of-plane oximato bridges are listed in Table 3. As shown in this table, a decrease of a leads to an increase of the ferromagnetic coupling [22]. In fact, a straight-line plot is obtained when representing J ' versus a for complexes 1-6, the expression being J ' ( c m -1 ) = 164.3- 1.53a (°). Notwithstanding this, it is clear that further magnetic and structural data are required to extend the range of the a values and to check the validity of this magneto-structural correlation.

In summary, by referring to simple molecules with com- mon structural features it is possible to account for the mag- netic properties of complex 1 in terms of its tetranuclear structure with the spin-structure pattern depicted in Scheme 3, where J is antiferromagnetic (through in-plane oximato bridge), while J ' is slightly ferromagnetic (through out-of-plane oximato bridge). Finally, it is also clear that this particular linear topology leads to a diamagnetic ground spin state in spite of the occurrence of a ferromagnetic interpair interaction, as illustrated by Scheme 3. Extension of this work to heterobimetallic complexes is in progress in our laboratory [36].

4. Supplementary material

Complete listings of atomic coordinates, bond distances and angles, hydrogen atom coordinates, anisotropic thermal parameters, least-squares planes and a listing of observed and calculated structure factors are available from the authors on request.

Acknowledgements

Financial support from the Spanish DGICYT through Pro- ject PB94-1002 and the Spanish-French Integrated Actions

R. Ruiz et al. / Inorganica Chimica Acta 268 (1998) 263-269 269

is g r a t e f u l l y a c k n o w l e d g e d . R. R u i z is i n d e b t e d to the M i n -

i s t e r io de E d u c a c i 6 n y C i e n c i a ( S p a i n ) f o r a p o s t d o c t o r a l

g ran t .

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